The one time I took a glass from a restaurant, I did so with the full knowledge and consent of the staff. I asked if I could buy it, and they said not to worry, I could just have it.

This was probably 20 years ago now at a Vietnamese restaurant in Chicago. They had a rather motley collection of water glasses, and I happened to get one that had a very old-school aesthetic, which was apparently originally from Caffarello's Restaurant and Cocktail Lounge, located "opposite main entrance of Chicago Municipal Airport" - which, Wikipedia informs me, was the original name of MDW (Midway) until 1949.

Yeah, I wanted that glass. I'm still thrilled by that find.